The cystic fibrosis transmembrane regulator (CFTR) is a protein of approximately 1480 amino acids made up of two repeated elements, each having six transmembrane segments and a nucleotide binding domain. Based on its predicted domain structure, CFTR is a member or a class of related proteins which includes the multi-drug resistance (MDR) or P-glycoprotein, bovine adenyl cyclase, the yeast STE6 protein, as well as several bacterial amino acid transport proteins. Proteins in this group, characteristically, are involved in pumping molecules into or out of cells. The CFTR has been postulated to regulate the outward flow of anions from epithelial cells in response to phosphorylation by cyclic AMP-dependent protein kinase or protein kinase C.
Cystic fibrosis (CF) is a lethal, hereditary, autosomal recessive disease which is caused by mutations in the gene coding for the CFTR Cl−-channel. By far the most common disease-causing mutation is the deletion of the codon for phenylalanine 508 (ΔF508) in the primary sequence of wild type CFTR. Over 90% of patients carry at least one allele of the ΔF508 CFTR mutant gene. The gene product from this mutant gene is a CFTR Cl−-channel that is poorly processed within the cell: most of the mutant protein is incorrectly or incompletely folded and becomes targeted for endoplasmic reticulum-associated degradation (ERAD). The few mutant Cl−-channels that pass the quality control or simply escape the ER before they are degraded will mature through the golgi and eventually be incorporated into the plasma membrane. These are thought to represent <5% of the level observed in cells expressing wild type CFTR, resulting in a commensurate low total whole-cell Cl−-conductance. In addition to the much lower number of channels in the plasma membrane, the open probability of the individual channel proteins is ˜3-fold reduced compared to wild type CFTR.
For over a decade, efforts have been ongoing to identify small molecule drugs that can restore the cell CFTR Cl−-conductance to levels high enough to ameliorate the effects of CF. These include correctors of ΔF508 CFTR, compounds that can improve the intracellular processing, and potentiators, compounds which increase the open probability of mutant CFTR channels at the cell surface.
A small molecule, dual-acting potentiator-corrector is expected to be of great benefit for the treatment of most CF patients. To date, it has proven difficult to develop compounds acting solely by correction of the intracellular processing that can sufficiently increase the number of channels in the cell surface to overcome the disease-causing deficiency in Cl−-conductance. On the other hand, potentiation, i.e., increase of open probability, of only the mutant channels at the cell surface will not sufficiently restore Cl−-conductance for most CF patients. A dual-acting potentiator-corrector molecule would mechanistically combine aspects of both corrector and potentiator compounds: the number of CFTR channels at the surface and the channel open probability are increased in parallel.